Spread spectrum technique transmits information over a wide range of frequencies. By spreading data over multiple frequencies robustness is enhanced. That said, spreading data over multiple frequencies requires greater bandwidth for the signal being modulated. This “spreading” is accomplished by modulating the signal twice, once with a carrier and then modulating the result with a Pseudorandom sequence. The spreading of the spectrum has some great benefits like resistance to jamming, resistance to interception, resistance to fading, and multiple access capabilities (e.g. CDMA). The direct spread spectrum system can also eliminate the impact of multipath distortion and reach a low SNR threshold.
CDMA system is usually recognized as an access technology for multiuser point-to-point communication. However, for a given system bandwidth, spectrum spreading limits peak user data rate to a relatively low value. Due to its low spectrum efficiency, the CDMA system was designed as an access technology for point-to-point communication, not for high-speed broadcasting or multicasting, which are point-to-multipoint services. Moreover, it has to apply a same modulation scheme, when multiplexing multiple data streams in CDMA. This significantly reduces flexibility, for example for hierarchical spectrum reuse.
Another method of data communication involves communication cells having known frequency characteristics. Here, a cell is associated with a carrier frequency and adjacent cells are each associated with other carrier frequencies such that intercell interference is limited. Unfortunately, even if the system only requires 4 carrier frequencies such a system reduces available bandwidth for each cell by 75%.
For broadcast communication, spectral efficiency and robustness are very important. That said, it has become apparent that differentiating broadcast signals—for example for local commercial insertion—is also of value. To date, this has been accomplished via wired communication or via local broadcasters broadcasting on their own frequencies such that the same information is broadcast numerous times by numerous broadcasters. Unfortunately, such a system is difficult to manage and is quite inefficient spectrally.
It would be advantageous to overcome some of the drawbacks of the prior art.